Laboratory experiments on core merging after a giant impact
Abstract:The fluid dynamics of core merging after giant impacts provides constraints on metal-silicate equilibration, core stratification, and possible dynamo initiation in the late stages of accretion. The energy released during giant impacts, such as those thought to have formed Earth's Moon and the crustal dichotomy on Mars, resulted in melting of the impactor and much or all of the protoplanet's mantle. Under these conditions, the liquid core of the impactor migrates through a fully liquid magma ocean and merges with the protoplanet's core.
In contrast with the laminar flow in numerical simulations, liquid impact experiments can produce turbulent entrainment and mixing, as expected during planetary formation. We present experiments on liquid blobs released into another liquid consisting of two immiscible layers, representing the magma ocean and protocore, respectively. The released liquid is immiscible with the upper layer, miscible with the lower layer, and denser than the upper layer. Our release conditions are compatible with a free-fall impact. When the released fluid density is equal to the density of the lower layer, we observe formation of a turbulent immiscible mixture in the upper layer, penetration of the immiscible mixture in the lower layer, and collapse into a gravity current that spreads between the upper and lower layers. By varying the density contrast between the released liquid and the liquid in the lower layer, we observe differences in the penetration depth and the extent of the stratification in the lower layer at the end of each experiment. Our results suggest that metal-silicate equilibration processes are not limited to the magma ocean but instead extend deep inside the protoplanet's core.